The enhancement of surface Raman scattering due to microscopic surface roughness has been the subject of much investigation during recent years. It has been noted by those skilled in the art that surface roughness appears to be necessary in order to enhance Raman scattering on surfaces that have deposits of small metal particles. See, for example, the article entitled "Electromagnetic Theory of Enhanced Raman Scattering by Molecules Adsorbed on Rough Surfaces," by J. Gersten and A. Nitzan, J. Chem. Physics, Vol. 73, No. 7, October 1980, pp. 3023-3037. A theoretical model for Raman scattering by a molecule adsorbed at the surface of a spherical metal particle was the subject of a paper entitled "Surface Enhanced Raman Scattering (SERS) by Molecules Adsorbed at Spherical Particles: Errata," by M. Kerker et al, Applied Optics, Vol. 19, No. 24, Dec. 15, 1980, pp. 4159-4174. It was suggested in that paper that the theory presented therein for spheres could also be extended to long cylinders and to spheroids as well as to layered particles.
Theoretical investigations have also been conducted to determine the effect of constructing a grating surface of the metal which is used to provide enhanced Raman scattering from molecules that are adsorbed on that surface. See, for example, the article entitled "Intensity of Raman Scattering From Molecules Adsorbed on a Metallic Grating," by S. S. Jha et al, Physical Review B, Vol. 22, No. 8, Oct. 15, 1980, pp. 3973-3982. In the above-identified articles that relate to enhanced Raman scattering, it was recognized that one of the primary contributions is the plasmon resonances that are associated with the microscopic bumps on the metal surface. It has also been recognized in the art that this type of local field enhancement can be used to provide second harmonic generation at a silver air interface. See, for example, the article entitled "Surface-Enhanced Second-Harmonic Generation," by C. K. Chen et al, Physical Review Letters, Vol. 46, No. 2, Jan. 12, 1981, pp. 145-148. As stated in the Chen et al article, local field enhancement should also be present for all nonlinear optical processes. The Chen et al article reported measurements on second harmonic generation from smooth and roughened surfaces of bulk samples of silver, copper and gold. Large enhancements due to surface roughness were said to be observed.
The mechanism underlying both types of enhancements, harmonic generation and Raman scattering, is the modification of the local fields inside and surrounding a small dielectric particle. This resonance has been the subject of many theoretical studies. See, for example, chapter 11 of the book entitled "Theory of Electric Polarization," by C. J. F. Bottcher, Elsevier Scientific Publishing Company, 2nd Edition, 1973. As pointed out in the book by Bottcher, the field inside the particle is a function of the external electric field, the complex dielectric function of the particle and a depolarization factor that is dependent on the shape of the particle. It has been recognized in the art that resonant amplification of the internal field will occur if the particle is properly shaped. It has also been recognized in connection with surface enhanced Raman scattering that the radiation emitted may also be enhanced in view of the fact that a plasma sphere may amplify the radiation field twice. See, for example, the article entitled "Surface Enhanced Raman Scattering," by S. L. McCall et al, Physics Letters, Vol. 77A, No. 5, June 9, 1980, pp. 381-383.